1. Field of the Invention
This application relates generally to feedwells for diluting and dispensing an influent slurry feed stream into a settling tank or basin, and more specifically to a self-diluting feedwell for diluting and dispensing an influent slurry feed stream into a thickener, clarifier, or settling tank without the need for a dilution pump or the addition of dilution liquor from an outside source.
2. State of the Art
Slurries or suspensions comprising liquids carrying suspended particles are typically subjected to a process called clarification to separate suspended particles from supernatant liquid. Typically, clarification is accomplished by continuously feeding an influent slurry or suspension feed stream into a settling tank or thickener, where suspended particles are allowed to gravity settle and form a sludge or thickened mud on the bottom of the tank. The thickened material is removed and further processed or disposed of, while the clarified liquid supernatant is either discharged, reused, or subjected to further clarification.
A feedwell is often used to introduce an influent feed stream flow into the settling tank or basin, as well as to provide a means of flocculating the feed stream and of distributing the feed stream into the tank in a non-turbulent manner. Disruption of the thickened material in the tank is minimized if the flow velocity of the influent feed stream is reduced before the influent enters to the tank. Typically, a feedwell dissipates the flow velocity by directing the feed stream into a circular or rectangular compartment in the center of the settling tank or basin which is separated from the contents of the settling tank or basin. The compartment is frequently mounted on a tower, column or pier in the center of the tank, but may be positioned around the perimeter of the tank or across the diameter of the tank. As the influent feed stream flows into the channel, much of the kinetic energy of the stream is dissipated due to the reduction in velocity head and the friction of the influent with the channel floor and walls. The feedwell may also contain a series of baffles to help dissipate the kinetic energy of the influent feed stream. The relatively quiescent influent is then allowed to enter the settling tank through ports in the sides or through an open bottom of the feedwell, or by allowing it to spill over the edge of the channel or from an influent weir into the settling tank.
The sedimentation process is sometimes expedited by adding a flocculating reagent to the influent before it enters the settling tank. The flocculating reagent typically has a polymeric molecular structure which agglomerates with suspended particles in the influent to form aggregate clusters called flocs. Flocs have a greater density than the discrete suspended particles, and settles to the floor of the tank in a more celeritous manner.
Several factors influence the effectiveness of flocculating reagents to agglomerate with suspended particles. The flocculating reagent must be mixed thoroughly with the influent and allowed ample time to agglomerate. If the concentration of suspended particles in the influent is low, the flocculating reagent may need to be stirred through the influent. This requires the addition of a stirring mechanism or flocculator to the settling tank. If the concentration of particles is high, the influent may need to be diluted for optimum flocculation to occur.
Dilution of an influent feed stream is currently accomplished by several methods. A pump may be employed to add a liquid, such as previously clarified liquor from the settling tank, to the influent feed stream. The feedwell channel can also be positioned below the liquid surface of the settling tank so that previously clarified liquor spills over the side of the feedwell and into the channel, diluting the feed stream. Problems with using a pump include the added expense and maintenance of the pump and power costs for operation thereof. The main drawback of the spill method is the lack of practical means to adjust the dilution ratio.
The present invention obviates many of the problems and expenses associated with prior art methodologies for diluting and flocculating an influent feed stream by using the principles of eductive flow and eductors to thoroughly and automatically mix a dilution liquor with an influent feed stream. The invention obviates the need for mechanical pumps or gravity feed apparatus to add a dilution liquor to an influent feed stream. The dilution ratio of influent to diluent can also be infinitely adjusted for different types and concentrations of influent suspensions and slurries. The invention also automatically compensates for changes in the influent flow rate so the dilution ratio remains substantially constant, independent of the influent flow rate.